The letters “NPK” stand for nitrogen, phosphorus, and potassium, the three primary nutrients plants need to grow. The numbers on the label indicate the ratio (by percentage) of nitrogen, phosphorus, and potassium in the fertilizer container.

Even if you do not see the letters N-P-K, but you see a set of three numbers, for example, 5-10-5, you can correctly assume it stands for 5% nitrogen, 10% phosphorus, and 5% potassium, always in that order. This fertilizer contains 20% nutrients; the remaining 80% is minor nutrients or fillers. Plants need about 16 nutrients; some they get from the air and water, and others are nutrients like iron, calcium, and chlorine from soil.1

Nitrogen

Nitrogen, plays a key role in a plant’s coloring and chlorophyll production, making it an important factor in leaf development. Fertilizers high in nitrogen are often used for grass or other plants where green foliage growth is more important than flowering.

Phosphorus

The middle number in an NPK series refers to the percentage of phosphorous in the fertilizer product. Phosphorus plays a key role in the growth of roots, blooming, and fruiting, which is why it is an essential nutrient for your plants in spring. Phosphorus contributes to many fundamental plant processes, such as rooting and seed formation.

Potassium

The final number gives the percentage of potassium in the product. Potassium contributes to the overall health and vigor of plants. It is known to help strengthen plants’ ability to resist disease, assist in the movement of water and nutrients in the plant, and can be especially important in areas that experience cold or dry weather.2

Other Ingredients

In addition to NPK, are micro=nutrients or trace elements like calcium, magnesium, iron, micronutrients, and even the percentage of organic matter and fillers. Although the minerals and micronutrients are less critical than the major nutrients, a good fertilizer product will include small amounts of other ingredients as well.